1. Field of the Invention
The present invention relates to an interface module, and more particularly to an interface module used to transmit a digital signal in a communication between LANs (Local Area Networks), computers, data terminals or the like.
2. Description of the Related Art
FIG. 9 is a circuit diagram showing a conventional interface circuit. A reference numeral 1 denotes a whole interface circuit. Reference numerals 2, 3 denote low pass filters. Reference numerals T1, T2 denote isolation transformers. Reference numerals T3, T4 denote common mode choke transformers. A reference symbol RE denotes a receiver. And, a reference symbol DR denotes a driver.
The whole interface circuit 1 is provided with a receiving slide, or section and a sending slide, or section. The receiving section is composed of the low pass filter 2, the isolation transformer T1 and the common mode choke transformer T3. The sending section is composed of the low pass filter 3, the isolation transformer T2 and the common mode choke transformer T4.
The low pass filter 2 removes noise (harmonic element) mixed on a transmission line. One end of a coil L1 and one end of a coil L2 are connected to each other through a capacitor C1. The other end of the coil L1 and the other end of the coil L2 are connected to each other through a capacitor C2. The low pass filter 3 removes harmonic element (noise at a normal mode). One end of a coil L3 and one end of a coil L4 are connected to each other through a capacitor C3. The other end of the coil L3 and the other end of the coil L4 are connected to each other through a capacitor C4.
The isolation transformers T1, T2 electrically separate between an input side and an output side (insulate between the input side and the output side). The common mode choke transformers T3, T4 remove common mode noises (noises having same phases on two lines).
An input signal on a receiving side input has a wave substantially identical to a sine wave, and is converted by the isolation transformer T1 after the common mode noise is removed by the common mode choke transformer T3. This signal is outputted to the receiving side output via the receiver RE after the noise mixed on the transmission line is removed through the low pass filter 2.
An input signal on a sending side input is a digital signal having a rectangular wave outputted by the driver DR. Then, this signal is inputted to the low pass filter 3 at the sending section. The wave of the signal inputted to this low pass filter 3 becomes substantially identical to the sine wave, since the harmonic element is removed. This signal is transmitted through the isolation transformer T2 to the common mode choke transformer T4, and then the common mode noise is removed and the signal is outputted to the sending side output.
Up to now, such an interface circuit is integrated as shown in FIG. 10. Namely, electronic parts, such as a bobbin around which coils L1, L2 of a low pass filter 2 are wound, a bobbin around which coils L3, L4 of a low pass filter 3 are wound, isolation transformers T1, T2, common mode choke transformers T3, T4 and the like are mounted on a front surface of a printed circuit board 101, and capacitors C1, C2, C3 and C4 (not shown in FIG. 10) of the low pass filters are mounted on a rear surface of the printed circuit board 101.
The interface module in which such an interface circuit is integrated is required to have an abrupt attenuation characteristic to attenuate signals not included in a predetermined frequency band because a fast response is necessitated.
In the conventional interface module, since the coils L1, L2, L3 and L4 constituting the low pass filter are formed in an open magnet circuit, an inputted signal having a high frequency is radiated as noise. Thus, in the conventional interface module, the noise radiated from the coil of the low pass filter is mixed into the isolation transformer or the common mode choke transformer, or the noise radiated from the coil of the one low pass filter is mixed into the other low pass filter. This results in a problem that it is impossible to obtain the sufficient attenuation characteristic.
In a case that other electronic parts which generate noise are mounted adjacently to the interface module, the external noise resulting from the electronic parts is mixed into the low pass filter, and thereby this results in the problem that it is impossible to obtain the sufficient attenuation characteristic. Moreover, in a case that the low pass filter 2 at the receiving section and the low pass filter 3 at the sending section are formed adjacently to each other, the coils L1, L2 constituting the low pass filter 2 and the coils L3, L4 constituting the low pass filter 3 at the sending section are electro-magnetically coupled to each other. Hence, this results in a problem that cross talk is generated.
In order to solve these problems, the low pass filter 2 at the receiving section, the low pass filter 3 at the sending section, the isolation transformers T1, T2 and the common mode choke transformers T3, T4 are formed so as to have intervals between them. However, this results in a problem that the size of the interface module becomes large. Moreover, even if having the above mentioned configuration, it is impossible to prevent the external noise from being mixed into the low pass filter. Hence, it is impossible to obtain the sufficient attenuation characteristic.
On the other hand, recently, it is desirable that such an interface module and the like are miniaturized, associated with the miniaturization of the apparatus comprising them. However, the conventional interface module can not be miniaturized because of the above mentioned problems.